Modern radiation therapy techniques include the use of Intensity Modulated Radiotherapy (“IMRT”), typically by means of an external radiation treatment system, such as a linear accelerator, equipped with a multileaf collimator (“MLC”). Use of multileaf collimators in general, and an IMRT field in particular, allows the radiologist to treat a patient from a given direction of incidence to the target while varying the shape and dose of the radiation beam, thereby providing greatly enhanced ability to deliver radiation to a target within a treatment volume while avoiding excess irradiation of nearby healthy tissue. However, the greater freedom that IMRT and other complex radiotherapy techniques, such as volumetric modulated arc therapy (VMAT), where the system gantry moves while radiation is delivered, and three-dimensional conformal radiotherapy (“3D conformal” or “3DCRT”), afford to radiologists has made the task of developing treatment plans more difficult. As used herein, the term radiotherapy should be broadly construed and is intended to include various techniques used to irradiate a patient, including use of photons (such as high energy x-rays and gamma rays) and particles (such as electron and proton beams). While modern linear accelerators use MLCs, other methods of providing conformal radiation to a target volume are known and are within the scope of the present invention.
Several techniques have been developed to create radiation treatment plans for IMRT or conformal radiation therapy. Generally, these techniques are directed to solving the “inverse” problem of determining the optimal combination of angles, radiation doses and MLC leaf movements to deliver the desired total radiation dose to the target, or possibly multiple targets, while minimizing irradiation of healthy tissue. This inverse problem is even more complex for developing arc therapy plans where the gantry is in motion while irradiating the target volume. Heretofore, radiation oncologists or other medical professionals, such as medical physicists and dosimetrists, have used algorithms to develop and optimize a radiation treatment plan.
Optimization is often used in IMRT to achieve a radiation treatment plan that can fulfill a set of clinical goals in terms of a set of quality indexes. Quality indexes may include statistical quantities of a dose distribution produced by a radiation treatment plan. For example, quality indexes may include maximum dose Dmax for a planned target volume (PTV), minimum dose Dmin for the PTV, mean dose Dmean for an organ at risk (OAR), percentage of the PTV receiving 100% of the prescribed dose V100% (i.e., dose coverage), and the like. Clinical goals may be expressed in terms of a set of threshold values for the set of quality indexes. For example, clinical goals may include the maximum dose Dmax for the PTV should be less than or equal to 105% of the prescribed dose (i.e., Dmax_PTV≤105%), the minimum dose Dmin for the PTV should be greater than or equal to 95% of the prescribed dose (i.e., Dmin_PTV≥95%), the mean dose Dmean to the OAR should be less than 20 Gy (i.e., Dmean_OAR≤20 Gy), the percentage of the PTV receiving 100% of the prescribed dose V100% should be greater than 95% (i.e., V100%>95%), and the like.
While a physician may be able to recognize a good treatment plan when such a plan has been obtained through optimization, it is difficult to specify a unique set of clinical goals prior to optimization. One possible approach is to use an initial set of clinical goals as a starting point to generate a reasonable candidate plan, and then interactively modify the dose distribution generated by the candidate plan to reach an optimal plan. One way of interactively modifying the plan is to directly make changes to the dose distributions, either in the three-dimensional fluence map or by modifying the dose volume histogram (DVH) curves, for various target structures and critical organs. In this approach, when a final optimal plan is reached, only the resultant dose distribution is recorded, which by itself may not clearly convey the physician's intent when she modified the dose distribution to reach the optimal plan. Therefore, it is desirable to have methods of interactively manipulating dose distribution in a treatment plan where the physician's intent may be preserved.